1. Field of the Invention
The present invention relates to an image forming apparatus and a process cartridge.
2. Discussion of the Background
A full color image forming apparatus generally employs one of the two typical systems.
One is referred to as a single or single-drum system in which one image bearing member is installed with four color development devices. In this system, a four color (cyan, magenta, yellow and black) toner image is formed on the image bearing member and transferred to a transfer medium directly or via an intermediate transfer body. Devices or members arranged around the image bearing member, which are, for example, a charging member, an irradiation device, a transfer device, a cleaning device and a fixing device, can be shared in the process of producing each color toner image. Therefore, this system can be small-sized and manufactured at a small cost in comparison with a tandem system described below.
The other is referred to as a tandem or tandem drum system, in which multiple image bearing members are provided. Typically, respective members or devices for charging, irradiation, development and cleaning are arranged around one drum (image bearing member) to form one electrophotographic element. A multiple number (typically four) of such electrophotographic elements are provided in an image forming apparatus. In this system, each electrophotographic element forms each corresponding single color toner image, which is sequentially transferred to a transfer medium to form a full color toner image.
The first advantage of this tandem system is high speed image formation since each color toner image is formed in parallel as described above. Therefore, the time to be taken for image formation in the tandem drum system is about a fourth in comparison with that in the single drum system so that the tandem drum system can form a full color toner image four times as fast as the single drum system. The second advantage is that each member or device including an image bearing member provided in one electrophotographic element has a substantially high durability. This is because one image bearing member in the single drum system is subject to the processes of charging, irradiation and development four times in total to form a full color toner image while each image bearing member in the tandem drum system is subject to each process only once.
However, the tandem drum system is large in size and expensive at cost. The image bearing member and each member or device arranged around the image bearing member have been reduced in size to deal with the size problem. This size reduction leads to saving material, which has a small but steady impact on the cost reduction in terms of the entire system. Also, this effect accompanies a new problem, which is about improvement of the sensitivity and stability of the image bearing member.
Generally, the chargeability and the sensitivity of an image bearing member easily deteriorate and the voltage at irradiated portion thereof tends to rise during repetitive use of the image bearing member. For example, as technologies to deal with the rise in the residual voltage of an image bearing member, unexamined published Japanese patent application No. (hereinafter referred to as JOP) H06-130688 and Japanese patent No. 3471163 describe a technology of containing a disphenoquinone based compound having a particular structure and a technology of containing a naphthoquinone derivative having a particular structure, respectively, together with a positive hole transport material in a photosensitive layer.
As described above, the main objective, which is also the main advantage, of this tandem drum system in which multiple electrophotographic elements are arranged in a full color image forming apparatus is high speed performance.
To take advantage of this feature, image formation processes should be performed at a high speed, which requires the improvement on responsiveness and stability of the photosensitivity of an image bearing member. That is, upon irradiation on the surface of a charged image bearing member, the image bearing member needs to optically attenuate quickly and maintain this optical attenuation characteristic during repetitive use.
Depending on the color ratio in produced images, only a particular color, e.g., black color, is repeatedly used. This especially occurs in the case of a full color image forming apparatus having a tandem drum system. This leads to a problem of usage bias (uneven frequency of usage) among the image bearing members in the full color image forming apparatus. This easily leads to deterioration of the optical attenuation property of an image bearing member relatively heavily used in comparison with the other image bearing members and results in variance in color tone of a full color image which is formed by overlapping colors during repetitive use.
In addition, oxidized gas produced by a charging device chemically affects the photosensitivity of an image bearing member, which leads to deterioration of the photosensitivity and production of abnormal images having defects such as deterioration of definition and image flow.
As described above, when the characteristics of an image bearing member are unstable over repetitive use and the image bearing member has been used for a long period of time, an image forming apparatus using the image bearing member produces images having a problem with the quality, resulting in variation in color tone or back ground fouling.
In addition, the inventors of the present invention describe a highly durable image forming apparatus having multiple electrophotographic elements each of which has a latent electrostatic image bearing member including a charge transport material having a particular imide structure, a latent electrostatic image formation device, a development device, a transfer device, etc. and an image formation method using the image forming apparatus in JOP 2007-264589 as a technology to stably produce full color images at a high speed during repetitive use.